This invention relates to power amplifiers and is more particularly directed to pulsed RF power amplifiers of the type in which several FETs or other amplifying devices are combined together to amplify an RF input signal. The invention is more specifically directed to a technique of controlling the bias levels of the amplifying devices so that the system gain is held constant despite increases in transistor die temperature.
Recently some circuit designers have begun to consider high voltage MOSFETs for use in power amplifiers as a means to reduce cost and size of the amplifier. These FETs, which are more commonly employed in switching power supplies, have a much more abrupt gate voltage to drain current characteristic than the RF power MOSFET. This makes a simple thermal compensation scheme difficult and costly to implement. These low cost FETs are also very sensitive to thermal variations, which can cause thermal runaway. If this sensitivity is not addressed adequately, destruction of the device can result. That is, threshold voltage change because of temperature change is a serious problem. The fact that the gate voltage to drain current characteristic is steeper and more abrupt as compared with RF power MOSFETs results in a faster, more extreme thermal runaway.
Additional problems arise where the high voltage MOSFETs are employed in a linear RF pulse power amplifier for low-band (5 to 25 MHz) magnetic resonance imaging (MRI). When these FETs are used, not only is it necessary to select an appropriate high voltage MOSFET, but care must be exercised in design of the push-pull circuitry for each pair of transistors. Thermal compensation of gate bias is needed to achieve dynamic linearity and gain stability. Thermal control of B+ supply, i.e. drain voltage, is required to achieve gain stability, and the cooling system must be optimally designed for management of heat, i.e., to cool the transistors evenly with highly efficient heat transfer.
The linear RF amplifier has to be designed so that each push-pull transistor pair amplifies evenly over the entire low band (5-15 MHz) with a high flatness characteristic around each given imaging frequency. The dynamic linearity must be maintained as high as possible over a wide range of pulse widths and duty cycles. That is, the output power to gain response over the specified dynamic range (40 dB) should be within a .+-.1.0 dB window.
Gain stability is defined as the variation of gain (for both long term and short term) at a specified peak RF output level. Gain stability should be maintained at .+-.0.2 dB for 15 minutes of operation, and at .+-.1.0 dB for 5000 hours of operation.
Phase stability is defined as variation of phase over the specified dynamic range and over time at a specific power level. The phase stability should be between .+-.2.degree. and .+-.5.degree., for short and long term, respectively.
Pulse droop is defined as the variation of peak RF output power over the pulse width for a specific output and duty cycle. Pulse droop should be within .+-.0.2 dB.
Pulse rise and fall times should be less than 25 .mu.sec, measured between the 10% and 90% levels of RF output.
The gated-on noise figure should be less than 27 dB for the overall system. This corresponds to less than -80 dBm/Hz gated-on output noise floor.
The gated-off noise figure should be no greater than 20 dB for the overall system, for a gated-off noise floor of less than -154 dBm/Hz.
The amplifier must be able to deliver the minimum specified power level into a variety of voltage standing wave ratio loads, or VSWRs. The amplifier must have a maximum output power capability into mismatched loads, so as to be useful for initial MRI system calibration.
At the present time, solid-state amplifiers utilize RF power MOSFETs which are designed and characterized for linear RF applications. The highest design operating frequency is less than the transistor's specified maximum frequency. The transistor's internal capacitances, C.sub.ISS, C.sub.RSS and C.sub.OSS are all low and have negligible effect on the overall source and load impedances. The RF power MOSFETs typically operate at 50 volts drain to source, and a pair in push-pull can provide peak output power of 400 watts, with a power gain of 13 dB. A typical MRI application requiring five kilowatts of peak RF power needs sixteen push-pull pairs.
On the other hand, high voltage MOSFETs, having a 400 volt breakdown characteristic and a 310 watt average dissipation capacity, can be operated at a nominal 85 volts drain to source, with a 10 dB power gain and 900 watts peak output power. This means that only eight push-pull pairs are needed to achieve a total peak output power of five kilowatts with a sufficient voltage breakdown margin to operate into high VSWRs. The high voltage MOSFET has greater than a 4:1 drain to source breakdown margin for 85 volts drain voltage to avoid voltage breakdown. Meanwhile, the RF input drive power level is kept the same as for the rated output regardless of load mismatch.
Therefore, because of the higher power and impedance mismatch capability of the high voltage MOSFETs, and also because of the lower cost of these than the RF power MOSFETs, any power amplifier that implements the high voltage MOSFETs would be extremely attractive.
Because of gain stability and dynamic linearity problems, however, these transistors cannot simply be substituted in place of the RF power MOSFETs. Instead, problems of drain bias stability, gain stability, and dynamic linearity must be taken into account. Means for dealing with these issues have not been addressed in the prior RF amplifier arts, even through the problems presented are by no means trivial.
Some of these problems have been addressed in commonly-assigned U.S. Pat. Appln. Ser. No. 08/275,124, now U.S. Pat. No. 5,477,188. The technique described therein is effective for intermediate term gain drift, but has been unable to respond to short term gain drifts.